Upon the breaching of the host physical barrier by intruding microorganisms, neutrophils, among all the leukocytes, are the first to influx into a focus of bacterial invasion for host defense (Kolaczkowska et al., Nat. Rev. Immunol., 13, 159-175 (2013)). Neutrophils used to be considered to function exclusively as the effector cells in the innate phase of the immune response. However, the old view was challenged since a growing body of evidence that neutrophils play a crucial role in framing immune response, both innate and adaptive immunity (Nathan, Nat. Rev. Immunol., 6, 173-182 (2006); Mantovani et al., Nat. Rev. Immunol., 11, 519-531 (2011)). For example, neutrophils are found to have a B cell-helper neutrophil population in the splenic marginal zone and these neutrophils can activate marginal zone B cells to secrete immunoglobulins against T cell-independent antigens (Puga et al. Nat. Immunol., 13, 170-180 (2012)).
The efficient recruitment of neutrophils depends on many signals, including N-formyl peptides, chemokines, complements and leukotrienes (Kolaczkowska et al., Nat. Rev. Immunol., 13, 159-175 (2013)). As the by-products of the protein translation in the invading bacteria, N-formyl peptides form molecular gradients originating from the bacteria in the infected tissue. And the gradients of N-formyl peptides signal neutrophils to migrate (i.e., chemotaxis) towards their targets while overriding other minor signals, such as IL-8 and MIP-2 (Kolaczkowska et al., Nat. Rev. Immunol., 13, 159-175 (2013)). Proposed in 1965 and confirmed in 1984, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLF) represents the best-known N-formyl peptide and one of the most well-established chemoattractants for neutrophils (Ye et al., Pharmacol. Rev., 61, 119-161 (2009)).
Recognized for its potential as a useful reagent to induce acute inflammation in vivo, fMLF, in the form of aqueous solution, has been injected subcutaneously (Gao et al., J. Exp. Med., 189, 657-662 (1999)), intravenously (Jagels et al., Blood, 85, 2900-2909 (1995)), intraplantarly (Rittner et al., PLoS Pathog., 5, e1000362 (2009)), intradermally (Feng et al., J. Exp. Med., 187, 903-915 (1998)) or just topically applied on the microvasculature (Oda et al., J. Leukoc. Biol., 52, 337-342 (1992)) to study the biology of neutrophils for various applications. Although the aqueous solution of fMLF is able to induce the accumulation of neutrophils, its effect is relatively weak and transient (2-6 hr) (Colditz et al., J. Immunol., 133, 2169-2173 (1984)).
Demonstrated in a recent work, the intratumoral injection of fMLF solution every two days after the inoculation of tumor cells slows down the tumor growth in a xenograft tumor model (Zhang et al., Lab. Invest., 76, 579-590 (1997)). Similarly, the daily intratumoral injection of another chemoattractant, chemerin, decreases the tumor growth (Pachynski et al., J. Exp. Med., 209, 1427-1435 (2012)). To maintain a meaningful local concentration of the chemoattractants, both studies required frequent intratumoral injections (Zhang et al., Lab. Invest., 76, 579-590 (1997); Pachynski et al., J. Exp. Med., 209, 1427-1435 (2012)).
A formulation of chemoattractant (e.g., fMLF) for prolonged release, not only acts as a useful tool to study the biology of neutrophils over long duration, but also holds promises for therapeutic applications, like cancer treatment. This potential has already led to explorations of different formulations of fMLF (Gauthier et al., Infect. Immun., 75, 5361-5367 (2007); Kress et al., Nat. Meth., 6, 905-909 (2009); Zhao et al., Biomaterials, 26, 5048-5063 (2005)), such as particles of fMLF in suspensions produced by sonication for studying neutrophil infiltration into pulmonary alveoli during murine pneumococcal pneumonia (Gauthier et al., Infect. Immun., 75, 5361-5367 (2007)), physically encapsulated N-formyl peptides in poly(lactic-co-glycolic acid) (PLGA) microbeads for inducing chemotaxis of neutrophils (Kress et al., Nat. Meth., 6, 905-909 (2009)), or human monocytes and monocyte-derived dendritic cells (DCs) (Zhao et al., Biomaterials, 26, 5048-5063 (2005)) in vitro.
Despite this progress, heterogeneous suspensions of fMLF particles are far from ideal for in vivo applications, and physical encapsulation using polymeric materials suffers from several limitations including burst release, low capacity for payload, and slow bioresorption of the polymeric materials. These limitations demand the development of new approaches to attract neutrophils in vivo.
The present invention overcomes these and other deficiencies in the art.